The Second Instalment – Week 3

In the First Instalment – Weeks 1-2 I had finished my week having got to grips with understanding the physics behind Lyman alpha haloes (LAHs), the sample and data I am investigating and also the significance of surface brightness (SB) profiles. The scripts have been developed in order to measure these SB profiles from the stacked images of various groupings of LAEs. 

I have settled on using a dr of 0.25 for my SB plots, such that a measurement of surface brightness is taken every 0.25 arc-seconds for annuli of this size. 

The SC4K catalogue uses 12 medium band filters, 4 narrow bands and 7 broadband filters to image the LAEs across a range of redshifts. The SB profiles will be measured and plotted for the 12 medium bands and 2 of the narrow bands such that there are 14 filters altogether. Since the LAEs in the SC4K catalogue stacks I am using are across 14 different filters, this corresponds to 14 different redshift slices ranging from z~2 to 6. 

This way, we are able to study not only the LAEs at a specific redshift, but also compare between redshifts and see if there is any evolution in the size of these LAHs, where the size is parametrised by reff, the scale-length.

This way, we are able to study not only the LAEs at a specific redshift, but also compare between redshifts and see if there is any evolution in the size of these LAHs, where the size is parametrised by the scale-length, r. 

One important aspect to look into is if the luminosity of a LAE correlates with its scale length, see Figure 1.

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Figure 1 – – The median Lyman alpha luminosity of each stack plotted against the measured scale-length from this stack. The measurements are shown for each of the four exponential fits performed, where the only difference is the radius range in which the fit is performed such that it either measures the central regions or the LAH.
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Figure 2 – The same as Figure 1 on a different scale and where the y errors correspond to the error in the determining the scale length from the exponential fit and the x ‘errors’ are shown to illustrate the range of LAE luminosities included in each stack.

There are stacks of LAEs for each redshift slice containing all the LAEs in that image using the SC4K catalogue. The median combined stacked LAE, that represents this sample of galaxies, has a luminosity equal to the median of all the luminosities of the LAEs in that stack. This is the luminosity I used to investigate a correlation with scale-length. 

The ‘x errors’ on these plots correspond to the minimum and maximum luminosity galaxy in the stack, such they span the entire luminosity range of the data. Each point in Figures 2 is a different stack, with its median luminosity plotted on the x axis and the measured scale-length on the y axis. The scale-length is measured by an exponential fit to the SB profile of the stack measured using annuli photometry discussed in the previous instalment. 

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Figure 3 – Median luminosity of each stack against the scale-length determined from the exponential fit to the SB measurements across a radius range of 1.5 – 5 arc-seconds in order to probe the size of the LAH. The colour bar shows the redshift of the stack.

Since all the stacks span a similar luminosity range and all the exponential fit measured scale-lengths are included in the same plot, see Figure 2, it is a very over-crowded and not very useful plot. Instead the same figure was created for each fit individually with the median luminosity and the range in luminosity, as well as being colour coded according to the redshift corresponding to each stack. These are much easier to read and interpret and an example of one these for the fit of 1.5 to 5 arc-seconds is shown here, Figure 2. It is important, when the best fit is decided using the point spread function (PSF), to re-make these plots for that fit. We already know we want to finish our fit at a radius of 5 arc-seconds, which corresponds to 40kpc at z=3, as beyond this the LAH is non-longer able to be detected above the noise. The issue is choosing the starting radius for the fit, which the PSF is needed for as discussed previously. This work will be completed next week and uploaded in the next instalment of the blog!  

It can also be useful to, instead of using the stacks for all the LAEs at a specific redshift, to use the already luminosity constrained stacks. So, for each filter, and hence redshift, the LAEs above a threshold (threshold luminosity of 43.0 ergs-1) have been stacked to produce a bright LAE stack, and then also those below the threshold to produce a faint LAE stack. Using these two stacks separately and measuring the SB profile, and hence the scale-length, is more useful for seeing any correlation between scale-length and luminosity over different redshifts.

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Figure 4 – The same as Figure 3 but for the stacks of only the bright LAEs, above a luminosity of 43.0 ergs-1.
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Figure 5 – The same as Figure 3 but for the stacks of only the faint LAEs, below a luminosity of 43.0 ergs-1.

The plots however, don’t appear to show any form of relationship between the luminosity and the scale-length of either the bright, faint or full sample of LAE’s; Figures 3, 4 and 5 respectively.  All of the Figures are plotted with a log scale in the y axis in order to see any small deviations or differences in scale-length more clearly. 

I mentioned that it would be possible, using the stacks in the different filters, to see if there is any evolution in the size of LAHs across redshift, and hence across cosmic time, instead of just looking at the size of LAHs on the whole.

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Figure 6 – Evolution of scale-length over redshift for stacks of all sources in each filter. The four different exponential fit ranges are included to highlight the differences between each.

Firstly, I investigated how the LAH’s scale-length (size) changes over redshift for all LAEs in the SC4K catalogue, such that each point in Figure 6, per fit, corresponds to a stack of all the sources at that redshift (those imaged in that corresponding filter). The scale-lengths are measured again for the different stacks using the four different exponential fits, where the difference is just the starting and finishing radii for the fit. All of these are plotted to highlight any differences or similarities between the different fits of scale-length across redshift. The black points, the fit from 0 to 2 arc-seconds is just telling us about the central light of the galaxy and not the LAH and so is not indicating the size of the LAH like the other fits are being used for.

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Figure 7 – Evolution of scale-length over redshift for stacks of the bright sources, above the threshold luminosity, in each filter.

We also look at the evolution of scale-length over redshift for the bright sources only for each filter, so measuring from the stack of the bright LAEs, which is a stack of the those with luminosities above the threshold luminosity of 43.0 ergs-1. This is because at high redshift we are unable to pick up fainter sources due to selection biases when observing. Therefore, looking at only the bright sources, which can be observed across all redshifts, we reduce this selection bias and are able to see if there is any evolution of the size of these bright sources across redshift and time, see Figure 7. 

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Figure 8 – Evolution of scale-length over redshift for AGN LAEs vs star forming LAEs.

Using the stacks of LAEs with AGN and those with no AGN, we can also investigate the difference in scale-length between AGN driven and star forming LAEs. Since, AGN and also forming O and B stars are the main producers of Lyman-alpha photons in galaxies, it can be important to separate the two and see if there are any differences between the LAHs produced by each and investigate any effects due to different mechanisms or whether the two mechanisms produce similar scale LAHs. It can also be important to separate the two in order to see if there are evolutions in scale-length over time for one mechanism of Lyman-alpha production and not the other. Do the two mechanisms produce the same type and scale of LAH? And if so, is this true for across all redshifts and time? Or are they producing similar LAHs originally but over time differences appear between the two? 

From Figure 8 we can see that the AGN powered LAHs are a lot larger in size than those produced by star formation only. It is difficult to see any evolution in the AGN powered LAHs sizes over redshift as our sample contains no LAEs containing AGN at the higher redshift and so we can only compare within a small redshift range. It is also important to note that the sample of AGN LAEs is also a lot smaller than the star forming LAEs. More data is needed to be able to see if there is an evolution of AGN powered LAHs sizes across time. However, our results do suggest that for the star forming produced LAHs, there is no redshift evolution over time. We also see that there is a clear difference in the size of LAHs produced by the two mechanisms. 

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Figure 9 – Scale-length over redshift for this work using exponential fits to measure the size of the LAH only for the LAEs. Other work is included to highlight similarities between our results and previous studies.

Finally, I compare the two different exponential fits to our data, specifically to the outer regions of the LAEs, with past literature. The two exponential fits to the SB profiles that start at 1.5 and 2 arc-seconds, are probing the LAH, and so can be compared to previous literature that also look into the haloes. Various studies are shown alongside our results in Figure 9. Firstly, Wisotzki et al 2015, presents the Lyα emission around individual star-forming galaxies at redshifts = 3–6 using an ultradeep exposure of the Hubble Deep Field South obtained with Multi Unit Spectroscopic Explorer (MUSE) on the ESO-VLT.

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The Very Larger Telescope (VLT) – ESO. Image credit:

In a follow up, Wisotzki et al 2018, MUSE data of 270 Lyα emitting galaxies at 3 < z < 6 were used, fitting circular Sersic models, to describe the Lyα haloes, rather than the exponential profile we use. Regardless of the different fitting methods, the results from this study appear to match up almost perfectly with our results, for an exponential fit from 1.5 arc seconds.

Next we compare with Momose et al 2014, that uses composite Subaru narrowband images to measure the scale lengths from the exponential profile fitted to the LAHs detected in the  z = 2.2 − 6.6 LAE samples. This study obtains scale lengths of ≃ 5 − 10 kpc at z = 2.2 − 5.7, and finds no evolution of scale lengths in this redshift range. The LAEs we observed also comes from using data collected with the Subaru telescope. Also, the LAHs were analysed in a similar way to to our work, over a similar redshift range, and also using exponential fits to the SB profiles. This makes comparisons really useful and our results, with the exponential fit starting from 1.5 arc seconds specifically, also appears to show no evolution in the scale-length over redshift, within the errors of our measurements.

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Subaru telescope – the 8.2-meter telescope of the National Astronomical Observatory of Japan. Image credit:

Finally we also compare with Leclercq et al 2017, which investigates lower luminosity sources, but spans a similar redshift range. Although the scale-lengths found in this study are, on average, lower than those in our work and other studies, probably due to their sample consisting of continuum-faint (−15 ≥ MUV ≥ −22) LAEs, they do however appear to show the same with regards to the evolution of scale-length over redshift.

Next week I plan to investigate and measure the point spread function (PSF) for the data I am using. Using this we can then determine a more accurate starting radius for our exponential fits and also convince ourselves that what we are measuring in our SB profiles is actually the LAH, and the emission is not simply coming from the central stars of the galaxy.

Emma Dodd

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